An optical fiber typically comprises a core portion and one or more cladding layers and permits the propagation of light rays along the core portion. For this reason, if two optical fibers are connected to one another, the cores of these two optical fibers should be precisely aligned. If the external shapes of two optical fibers coincide with one another, but either or both of the cores have eccentricity, the connection loss increases due to the discrepancy between the positions of these cores. For this reason, the preform as a starting material to be drawn into an optical fiber should comprise a core portion with minimal eccentricity. In the production of an optical fiber and the management thereof, it is very important to inspect an optical fiber preform as a starting material and to thus select an optical fiber preform comprising a core portion having low eccentricity from the foregoing standpoint. Additional geometric properties, in addition to eccentricity, are also important, including the outer diameter of the core and cladding, as well as any ovality.
While such properties could be visually inspected and measured at a given cross-section of an optical fiber preform, such a methodology is cumbersome and slow, requiring taking multiple cross-sections of a fiber preform along its length, and not at all suited to commercial production of fiber preforms. Moreover, as such methodologies would require taking multiple cross-sections of a preform along its length, they would effectively destroy the preform. Alternatively, an optical fiber preform could be immersed in a refractive index matching oil and scanned with a laser beam to determine its refractive index and geometric properties but the necessity and the cost of cleaning the preforms after oil immersion are not desirable for the low cost commercial production of optical preforms. The invention provides a low cost method for measuring the geometrical properties of the fiber preform in air and in a non-destructive manner and furthermore allows for additional cost reduction by eliminating the cost of fiber draw from parts of the preforms which do not satisfy the geometrical specifications.
Accordingly, methodologies for determining geometric properties of optical fiber preforms in commercial production must be able to quickly provide such determinations without damaging or altering the preform. Currently, no such low cost, robust and accurate inspection and measurement methodologies exist. Previous methodologies to determine the geometrical properties of optical fiber preforms require polarized light, or complex, costly and difficult to handle operations such as immersion in a material (e.g., a refractive index matching oil) having the same index or refraction as the cladding material of the preform. These alternative methods have errors related to sensitivity to local light environment and require handling and operation not compatible with low cost production inspection and measurement process.